The present invention relates to a method for the induction heating of steel pipe, and more particularly to a method for performing uniform induction heating of the steel pipe in its axial direction while moving the steel pipe in its axial direction.
In performing the induction heating of the long steel pipe with commercial electric power, a method of heating the steel pipe while shifting the steel pipe in the coil of the fixed induction furnace is usually employed. As one factor in obtaining a satisfactory heating effect, the soaking of the steel pipe in its longitudinal direction is required in this case.
A matter that becomes particularly problematic in the soaking of the steel pipe in its longitudinal direction, is that a temperature differential exists between the middle portion and the pipe end. For example, when the heating treatments such as hardening and tempering are performed, the pipe end is not controlled within the desired temperature differential with respect to the middle portion of the steel pipe, with the result that a satisfactory heat treatment cannot be performed and adverse effects arise such as the fact that the desired properties of material and shape at the pipe end are not obtained and also that deterioration of yield due to the discarding of the inferior portion occurs.
The temperature difference between the middle portion and the pipe end as mentioned in the foregoing is caused by the difference in the heat radiation of the pipe end and the middle portion as well as the temperature gradient accompanied by the transfer heating, and in normal cases, such problems cannot be avoided.
To eliminate the temperature difference, a variety of methods have been proposed such as an auxiliary heating method using an auxiliary heating apparatus, a continuous processing method, a method of providing a soaking zone, and a method of adjusting electric power, but each method has its specific problem as will be described in the following and therefore none of them is an effective means for solving the problems.
(a) Auxiliary heating method
In this method, auxiliary heating is applied only to the temperature lowered portion of the pipe end before or after the main heating, but the conditions of the main heating operate delicately on the auxiliary heating and it requires complicated adjustments whereby the stabilized conditions cannot be obtained. PA1 This method involves continuously heating the front and rear steel pipes which are abutted at their ends; the dimension of the pipe end or the perpendicularity to the pipe axis are irregular and as a result, the abutted condition where the pipe ends of the front and rear pipes are not tightly abutted, cannot be obtained. PA1 Also, there has been employed a similar method of connecting a dummy piece to the front and rear steel pipes, but disadvantages such as the loss of the dummy piece or the deteroriation of the work efficiency are great. PA1 This method generally employs an ordinary heating furnace, but in case of the internal heat generating system of the steel material, as in the case of he induction heating, it requires a separate installation for the indirect heating (for example, gas furnace) and it is meaningless as the heating characteristics is spoiled. PA1 (d) Power adjusting method PA1 This involves detecting temperature in the vicinity of the pipe end during the heating operation, and compensating for the temperature fluctuation by the adjustment of the heating electric power to be impressed, but it is not economical due to the complicated control requirements and equipment according to a variety of conditions.
(b) Continuous processing method
(c) Soaking zone method
Under the foregoing circumstances, in the induction heating of the steel pipes, the development of a simple heating method of great utility which solves the foregoing problems has been desired.